1. Field of the Invention
The present invention relates to a liquid developing unit employed in an image forming apparatus, and more particularly, to a liquid developing unit which is capable of achieving a high binary ink development (BID) with a simple structure.
2. Description of the Related Art
A conventional image forming apparatus includes a charger increasing a potential of a surface of a photosensitive belt, an exposing unit forming a latent electrostatic image by radiating a beam onto the charged photosensitive belt, a developing unit developing an image by supplying a developing solution to the latent electrostatic image, a dryer drying a carrier from the developing solution to form the image, a transferring unit transferring the dried image onto paper, and a fixing unit fixing the transferred image on the paper by applying heat or pressure to the transferred image of the paper.
Here, a conventional developing unit 10 is shown in FIG. 1. Referring to FIG. 1, the conventional developing unit 10 includes a developing solution supply unit 12 supplying a developing solution 30, a developing roller 13, whose lower half is soaked (immersed) in the developing solution 30, supplying the developing solution 30 to a photosensitive drum 11, the photosensitive drum 11 contacting the developing roller 13 and developing a latent electrostatic image with the developing solution 30 to form an image, a depositing roller 15 contacting the developing roller 13 to deposit the developing solution 30 on the developing roller 13, and a cleaning roller 17 rotating while contacting the developing roller 13 to clean the developing solution 30 remaining on the developing roller 13 after the development of the latent electrostatic image.
A developing process in the conventional developing unit 10 includes depositing the developing solution 30 on the developing roller 13 using the depositing roller 15, metering and squeezing an ink layer 20 deposited on the developing roller 13 using a metering blade 19, developing the latent electrostatic image with the developing solution 30 transferred from the developing roller 13 onto the photosensitive drum 11 by a first potential difference in a developing gap formed between the developing roller 13 and the photosensitive drum 11, pressing toner (developing solution 30) and fixing the developed image on the paper, and cleaning the developing roller 13 by removing the developing solution 30 remaining on the developing roller 13 using the cleaning roller 17 after the development of the latent electrostatic image.
First, a principle of a depositing operation is simply shown in a circle A. An electric field E is formed between a surface 29 of the depositing roller 15 and the developing roller 13 having a resistance layer 23 and a conductive layer 21 by a difference between a first voltage supplied to the depositing roller 15 and a second voltage supplied to the developing roller 13, and ink particles (ink) of the developing solution 30 having charge “q” are transferred to and deposited on the developing roller 13 from the depositing roller 15 by a Coulomb force (F=qE).
A metering operation after the depositing operation is shown in a circle B. The metering of the ink layer 20 forms a uniform thickness (mass per area (M/A)) of the ink layer 20, which is deposited on the developing roller 13 by a second potential difference between the depositing roller 15 and the developing roller 13. The metering of the ink layer 20 also squeezes a carrier of the developing solution 30 and applies physical pressure to the ink layer 20 using the metering blade 19 to form a high concentration (% solid) layer as shown in the circle B.
As shown in a circle C, a developing operation after the metering operation is performed in a developing gap between the photosensitive drum 11 and the developing roller 13. The ink particles (ink) of the developing solution 30 are charged by the first potential difference between a surface 25 of the photosensitive drum 11 and the developing roller 13 having the conductive layer 21 and the resistance layer 23 stacked on the conductive layer 21. The ink particles move to the surface 25 of the photosensitive drum 11 from the resistance layer 23 of the developing roller 13.
Last, as shown in a circle D, the developing solution 30 remaining on the developing roller 13 after the development of the latent electrostatic image is cleaned by the cleaning roller 17 having a sponge shape, thereby completing the above developing process.
In the conventional developing unit 10, the developing solution 30 having ink having a high concentration and a carrier is transferred through the developing gap in the developing operation, and thus a surplus carrier is captured (recollected) from the ink layer 20 of the developing roller 20 into the developing solution supply unit 12 in the metering operation, and the concentration of the ink of the developing solution 30 contained in the developing solution supply unit 12 continuously decreases. Therefore, an amount and concentration of the toner (ink) containing in the ink layer 20 formed on the developing roller 13 after the metering operation cannot be irrelevant to the concentration of the developing solution 30 contained in the developing solution supply unit 12 even though the metering operation is performed accurately. For this reason, the concentration of the ink of the developing solution 30 varies, and 100% of a binary ink development (BID, a developing efficiency of the developing unit) cannot be achieved.
Also, in a case that the 100% of the BID is not achieved during the development of the latent electrostatic image, the cleaning roller 17 being overloaded to remove residual toner (ink) from the developing roller 13 by an electric force should be installed in the developing solution supply unit 12. However, since the cleaning roller 17 contacts the developing roller 13 which requires an accurate driving speed and applies a rotation load to the developing roller 13, the image deteriorates.